<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

    <script>
        let str = 'addddasfafdssadaf'
        var index = str.indexOf('a')
        var num = 0
        while (index !== -1) {
            console.log(index);
            num++
            index = str.indexOf('a', index + 1)
        }
        console.log('a出现的次数',num);
    </script>
</body>

</html>